1. Field of the Invention
The present invention relates to an exhaust-gas-recirculation system for controlling the emission of noxious components, particularly nitrogen oxides (NOx) of the exhaust gas emitted from diesel engines.
2. Description of the Prior Art
It is well known to use exhaust-gas-recirculation system (hereinafter referred to as "E.G.R. system") for recirculating a part of the exhaust gas to the intake system of an internal combustion engine to control the emission of noxious components, particularly nitrogen oxides (NOx) of the exhaust gas from the internal combustion engine. In case of emission control in diesel engines, it is a common measure to reduce the E.G.R. ratio (the ratio of flow rate of recirculated exhaust gas to the sum of the flow rate of intake air and the flow rate of recirculated exhaust gas) as the load applied to the engine is increased. To this end, conventionally, the E.G.R. ratio has been controlled in relation to the amount of depression of the accelerator pedal. This permits the control of the E.G.R. ratio in response to the change in level of load if the accelerator pedal is operated to compensate for the change in level of the load to maintain a constant engine speed. However, if the automobile runs with the constant position of the accelerator pedal irrespective of the change in level of the load, the engine speed is changed in response to the change in level of the load, while the E.G.R. ratio is kept unchanged.
Thus, the conventional system could not provide the optimum control of the E.G.R. ratio in response to the change in the level of load over all modes of engine control, although the control of E.G.R. ratio is made satisfactorily in some specific modes of engine control.